1. Field of the Invention
The present invention relates to an exposure apparatus, and more particularly to an exposure apparatus for exposing a photosensitive body by carrying out the main scanning of the photosensitive body by deflecting a light beam emitted from a light source.
2. Description of Related Art
Exposure apparatuses have conventionally been employed in image forming apparatuses such as copying machines, fax machines and laser printers. Such an exposure apparatus carries out the main scanning of the surface of a drum-like photosensitive body with a laser beam corresponding to image information using a light deflecting means such as a polygon mirror, and carries out subscanning by rotating the photosensitive body in the subscanning direction. Thus, an electrostatic latent image corresponding to the image information is formed on the photosensitive body.
Recently, since the printing speed of the image forming apparatuses has increased, the polygon mirror rotation rate in an under filled optical system is reaching its limit. Accordingly, over filled optical systems have been employed. In the over filled optical system, the laser beam is incident upon a plurality of reflection planes of a polygon mirror simultaneously by making the scanning direction length of the individual reflection planes of the polygon mirror smaller than the scanning direction diameter of the laser beam incident upon the polygon mirror. Compared with the under filled optical system, the over filled optical system can greatly reduce the scanning direction length of the reflection planes required for producing a prescribed magnitude spotlight on the photosensitive body. In addition, the over filled optical system can provide a greater number of reflection planes to the polygon mirror of the same diameter than the under filled optical system. Thus, the over filled optical system can rotate the polygon mirror at a lower speed than the under filled optical system. Besides, the over filled optical system can employ a polygon mirror driving unit with smaller power consumption.
In the over filled optical system, however, each reflection plane of the polygon mirror reflects part of the incident laser beam as described before. Accordingly, the light quantity incident upon the reflection plane of the polygon mirror varies depending on the angle of the polygon mirror. FIG. 10 is a graph showing the conditions. The distribution of the light quantity incident upon the photosensitive body is as illustrated in FIG. 10. More specifically, the light quantity takes the highest value at the center of the photosensitive body, and gradually decreases toward the two edges. The variations in the light quantity distribution can have adverse effect on the image quality.
To solve the problem, Japanese patent application laid-open No. 09-197316/1997 discloses a technique that measures the light quantities at points on a scanning plane on the photosensitive body, measures fluctuations on the scanning plane, and stores the fluctuations. According to the stored values, it corrects the light quantity of the laser beam, thereby controlling the light quantity on the scanning plane in such a manner as to become nearly constant.
However, to enrich media, to improve efficiency of developing resources, or to speedup product development, it is required to apply a single exposure apparatus to a variety of apparatuses such as a laser printer or MFP (multifunction peripheral). Thus, it becomes necessary for individual exposure apparatuses to cope with various specifications.
In particular, to cope with the various specifications, this type of the exposure apparatus has a problem of the rotation rate of the polygon mirror and the laser light quantity. As for the rotation of the polygon mirror, such an apparatus that detects the rotation rate according to the detection period of a horizontal synchronizing signal detecting means and controls the rotation will be able to vary the rotation rate inexpensively and easily.
As for the laser light quantity also, the conventional under filled scanner can easily vary the light quantity by varying the target light quantity of APC (automatic power control). In contrast, the over filled optical system must carryout the light quantity correction during a single scanning.
However, as described in Japanese Patent Application Laid-open No. 09-197316/1997, the conventional apparatuses do not cope with a plurality of scanning light quantities. Thus, an exposure apparatus capable of coping with various specifications has not been implemented.